William A. Douthwaite

The effect of an artificially elevated intraocular pressure on the central corneal curvature

Twenty-three optometry students with normal corneal condition were recruited. The intraocular pressure and central corneal curvature of the right eye were measured in a sitting and a 30 degrees head-down posture. The mean (standard deviation) IOPs before and during posture change were 15.6 (2.4) mmHg and 22.1 (2.3) mmHg respectively. This 6.5 mmHg mean rise in IOP was found to be statistically significant which is similar to the results from previous studies. The mean changes in radius of corneal curvature and the orientation of the axis of the vertical principal meridian were 0.02 mm (SD 0.025 mm) and 2.4 degrees (SD 10.4 degrees) respectively. No significant variation was demonstrated on these keratometric results due to the 30 degrees head-down posture. The maximum change in radius of curvature was only 0.055 mm for one subject. Perhaps this amount of pressure rise was not sufficient enough to distort the corneal surface centrally. Another possibility could be an even distribution of the elevated pressure around the cornea, or the distribution of pressure is not even but could not be revealed by a conventional keratometer.

Corneal asphericity and refractive error

The relationship between corneal curvature measured by classical keratometry and refractive error raises the question of a possible systematic relationship between refractive error and the degree of corneal asphericity. A recomputation and analysis of previously published data leads to the conclusion that the different refractive groups have similar values of the parameter ‘p’ and differ only in the apical radius of the cornea.

The Relation between Invasive and Noninvasive Tear Break-Up Time

TBUT and NITBUT (tear break-up time and noninvasive tear break-up time, respectively) were measured on four groups of subjects from different countries (two groups of Caucasians, two groups of Chinese). No significant difference was found in either TBUT or NITBUT values among the four groups of subjects. Gender of the subjects was not a factor affecting the TBUT or NITBUT values. The distributions of both NITBUT and TBUT were non- Gaussian and the median value of either parameter was lower than those previously reported. The NITBUT values were significantly higher than the TBUT values in both Caucasian and Chinese subjects. An investigation into the extent of agreement between the TBUT and NITBUT methods of assessing tear stability indicated that these methods do not agree sufficiently closely (that is, tear stability values measured with the NITBUT technique do not agree with those measured with the TBUT technique); short average tear break-up values are associated with small differences between the measures, whereas long average tear break-up values are associated with large differences between the measures.

Does the change of anterior chamber depth or/and episcleral venous pressure cause intraocular pressure change in postural variation?

Purpose. Previous studies have found that the intraocular pressure (IOP) variation from postural change is due to the obstruction of aqueous outflow by an increase in episcleral venous pressure. This study investigated if any shift of anterior lens position from postural variation would be another contributing factor. Methods. Thirty-three Chinese subjects were recruited with their IOP and anterior chamber depth (ACD) measured in the sitting, supine, and prone postures. The IOP was measured using a Pulsair 2000 noncontact tonometer and ACD with a Nidek US-2000 EchoScan unit. Results. The highest IOP was obtained in the prone position and this value was significantly different from the IOP obtained in other postures, whereas there was no significant difference in ACD. Conclusions. Because no significant variation in ACD was demonstrated, the prone and supine IOP variation could be due to something other than the change in lens position. However, a higher IOP in the prone position rather than in the supine position also suggests that it is not merely the episcleral venous pressure causing the IOP change. Investigation of the entire iris profile at different postures would be more informative in future studies.

The EyeSys videokeratoscopic assessment of apical radius and p-value in the normal human cornea.

The EyeSys videokeratoscope was used to assess the corneal topography in 98 subjects. Scatterplots of distance squared versus radius squared were plotted for the near horizontal and near vertical principal meridians of the two eyes. The regression lines allowed calculation of the surface apical radius and the p‐value. The group average apical radius was 7.93  mm (horizontal) and 7.78  mm (vertical). The group average p‐value was 0.76 (horizontal) and 0.82 (vertical). Both apical radius and p‐value were similar when comparing the two eyes for both the horizontal and the vertical meridians. The two meridians in a single eye, however, had different values for both apical radius and p‐value. Male apical radii were longer than those of females but the p‐values were the same. There is no apparent association between age and either apical radius or p‐value for the subjects used in this study. The asphericity of the cornea does not show any apparent association with corneal curvature in this group of subjects.

Mathematical models of the general corneal surface

Although the curvature of the general corneas two principal meridians can be acceptably modelled (within limits) by ellipses, this is insufficient to delineate the overall corneal surface. Applying different assumptions in either case, two corneal models arc advanced which interpolate x, y, z points over the whole corneal surface, dependent only on the specifications of the plane curves of the principal meridians.

The corneal-thickness profile in Hong Kong Chinese.

PURPOSE This study investigated the topographic corneal thickness in four groups of Hong Kong Chinese. METHODS The corneal thickness was determined with an ultrasound pachometer mounted on a X-Y plate. A head rest was used to reduce further any movement from the subject during the measurement. After the central cornea was measured, the peripheral cornea was determined on nasal, temporal, superior, and inferior regions, 2 and 5 mm from the corneal center. RESULTS The corneal thickness was not significantly different between the right and left eyes. For the right eye, there was no significant thickness difference for the four quadrants at a midperipheral and peripheral region, respectively. There was a general thinning of the corneal thickness at all regions from aging but no difference between the genders. CONCLUSION The corneal thickness in our subjects was similar to that in other studies. The mean central corneal thickness varied from 541.7 (m to 560.8 microm, dependent on age. This study provides some information for the future studies of Chinese corneal thickness.

EyeSys corneal topography measurement applied to calibrated ellipsoidal convex surfaces.

AIMS/BACKGROUND--This study was carried out to assess the accuracy of the EyeSys videokeratoscope by using convex ellipsoidal surfaces of known form. METHODS--PMMA convex ellipsoidal buttons were calibrated using Form Talysurf analysis which allowed subsequent calculation of the vertex radius and p value of the surface. The EyeSys videokeratoscope was used to examine the same ellipsoids. The tabular data provided by the instrument software were used to plot a graph of r2 versus y2 where r is the measured radius at y, the distance from the corneal point being measured to the surface vertex. The intercept on the ordinate of this graph gives the vertex radius and the slope the p value. The results arising from the Talysurf and the EyeSys techniques were compared. RESULTS--The EyeSys videokeratoscope gave readings for both vertex radius and p value that were higher than those of the Talysurf analysis. The vertex radius was around 0.1 mm greater. The p value results were similar by the two methods for p values around unity but the EyeSys results were higher and the discrepancy increased as the p value approached that of a paraboloid. CONCLUSIONS--Although the videokeratoscope may be useful in comparative studies of the cornea, there must be some doubt about the absolute values displayed. The disagreement is sufficiently large to suggest that the instrument may not be accurate enough for contact lens fitting purposes.

Corneal shape of Hong Kong-Chinese.

Corneal asphericity (the p-value, p) and apical corneal radius (r0) were calculated for 63 Hong Kong-Chinese (39 male and 24 female), using the Topographic Modeling System (TMS-1). The values of p and r0 in, and between, the two principal meridians were compared, and the effects of refractive error and gender were also investigated. The mean +/- SD r0 and p along the flattest meridian of the right eye were 7.82 +/- 0.26 mm and 0.78 +/- 0.12 respectively. The mean +/- SD r0 and p along the steepest meridian of the right eye were 7.64 +/- 0.26 mm and 0.83 +/- 0.15 respectively. Meridional variations were found in r0 and in p. There was no correlation between p and r0 along the two principal meridians. p was not significantly correlated to the refractive error but r0 was. Our results agree with previous findings that while males have longer r0 than females, their p are not significantly different.

Binocular summation in visually evoked responses and visual acuity

Monocular and binocular transient visually evoked responses (VER) were recorded on 50 adult subjects using a pattern reversal stimulus (check size 5.5 min arc). The peak‐to‐trough amplitude of the VER wave was measured and compared with the subjective visual acuity (Landolt C). The binocular amplitude enhancement over the monocular amplitude was around 26% across the acuity range studied, whereas the mean binocular subjective visual acuity improvement was around 11.3%, but this varied from 1% at the high acuity end to 17% at the low acuity end of the range.